Associative Polymers in Aqueous Media - American Chemical Society

Chapter 20

Determination of the Thickening Mechanism of a Hydrophobically Modified Alkali Soluble Emulsion Using Dynamic Viscosity Measurements Downloaded by STANFORD UNIV GREEN LIBR on September 22, 2012 | http://pubs.acs.org Publication Date: August 10, 2000 | doi: 10.1021/bk-2000-0765.ch020

C. M . Miller, K. R. Olesen, and G. D. Shay

1

UCAR Emulsion Systems, Union Carbide Corporation, Cary, NC 27511

The viscosity of a HASE latex was measured using a reactor calorimeter by developing a relationship between a measured variable closely related to agitator torque and viscosity. A particular advantage of this technique was its ability to continuously monitor the solution viscosity as the pH was adjusted by the addition of sodium hydroxide. Thus, the technique permitted continuous monitoring of the dynamics of the neutralization process, and particularly the equilibration behavior as a function of degree of neutralization. The results of this study show that the neutralization behavior of a HASE thickener is similar to that reported previously for ASE thickeners. Specifically, the viscosity of any given thickener is a function of degree of neutralization and concentration of the thickener. Furthermore, at sufficiently high concentrations a pronounced viscosity spike is observed at degrees of neutralization between 45 and 55%. Rheology and light scattering measurements suggest that the cause of this peak is predominantly due to the large increase in hydrodynamic volume of the latex particles in their highly water-swollen state immediately prior to their dissolution. Dynamic equilibration experiments revealed that the equilibration rate of a HASE thickener is a strong function of degree of neutralization, with the longest equilibration times occurring between 25 and 44% neutralization.

Corresponding author. 338

© 2000 American Chemical Society

In Associative Polymers in Aqueous Media; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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Introduction Alkali-swellable and alkali-soluble thickeners (AST's) are carboxyl functional polymers produced by free radical polymerization of ethylenically unsaturated monomers (/). These polymers are substantially insoluble in water at a low pH, however, at higher degrees of ionization (higher pH) they become swellable or soluble in water and thus exhibit thickening behavior. As a result of their pH dependent solubility in water, AST polymers can be prepared either as polymer solutions at high pH or as latexes at low pH. Among these, the latex AST's are the most industrially important due to their much lower viscosity in the latex state. In their ionic form, AST latexes are generally broken down into two classes of materials, either alkali-soluble (or swellable) emulsions (ASE's) or hydrophobically modified alkali-soluble (or swellable) emulsions (HASE's). The difference between these two classes of latexes pertains to the nature of the polymer backbone, which in turn impacts the thickening mechanism. For ASE's, the polymer backbone is generally comprised of acrylic monomers and carboxylic acid monomers, and thus at high pH these materials swell or dissolve in water to thicken by an intermolecular entanglement mechanism, similar to that observed for conventional polymers in organic medium. Electrostatic repulsion of the carboxylate anions in the ASE promotes molecular coil expansion increasing the hydrodynamic volume and chain entanglement. For HASE's, the polymer backbone is essentially the same, however in this case the acrylic polymer backbone is modified with hydrophobically terminated ethoxylated macromonomers which provide a secondary tmckening mechanism. For these polymers the mechanism of thickening by intermolecular entanglement is retained, as described above, however, additional thickening is derived from a micelle-like association of hydrophobic moieties along the polymer backbone. This can impart unique rheological properties to HASE thickeners compared to the ASE thickeners. Regardless of the thickening mechanism, ASE and HASE latexes find great utility in a variety of practical applications including architectural coatings, carpet backing, printing inks, paper coatings, and adhesives. In these applications, copolymers and terpolymers containing predominately ethyl acrylate and methacrylic acid are most frequently used. As a consequence, a large number of papers and patents have been published describing the synthesis and rheological properties of AST's. In addition to the synthesis and rheology of AST's, the mechanism for transition from a water insoluble latex to a water soluble polymer has also received a good deal of attention in the literature, however, in this case most work has been done with ASE's and relatively little work has been published on HASE Thickeners. The remainder of this introduction reviews some of the work on the mechanism for the ASE latex to polymer-solution transition. Fordyce et al. (2) reported a marked viscosity increase upon neutralization of alkali-soluble methacrylic acid co alkyl acrylate emulsion copolymers (ASEs), where the properties of the alkali responsive thickeners examined depended on the ratio of monomer used, the molecular weight, and the extent of crosslinking present. Fordyce et al. also observed viscosity maxima at degrees of neutralization less than 100%, i.e., where thefraction(a) of carboxyl groups neutralized was in the range of α = 0.100.80. They concluded that at the viscosity peak, a true solution does not exist, but instead, a highly swollen insoluble state contributes to the high solution viscosity the magnitude of which was dependent on polymer concentration. Finally, in this work it was also reported that the solubilization of the ASE thickeners occurred at a high rate



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and was controlled almost entirely by the rate at which the alkali could be uniformly distributed through the emulsion, although it is unclear how this conclusion was arrived at. Yudelson and Mack (5) also observed extraordinary viscosity maxima at degrees of ionization substantially less than 100% for certain acrylic acid co alkyl acrylate copolymers which had been prepared by solution polymerization in nonaqueous media. The pH at which the maxima occurred for the aqueous dispersion of dried polymer was referred to as the "gelation pH". Because the gelation behavior was only observed at concentrations above some critical value, they concluded that the gelation phenomenon must be due to intermolecular interactions. Attractive forces due to hydrogen bonding of carboxyl groups with ester groups were in balance with repulsive forces due to coulombic repulsion of the carboxyl anions. In a comprehensive examination of latex to solution transition, Verbrugge (4,5) identified three general profiles of viscosity as a function of neutralization degree for a large number of methacrylic acid containing copolymer emulsions. The profile shape was attributed to a combined variety of factors foremost of which was the %MAA in the copolymer, followed by the relative copolymer hydrophilicity, and lastly the copolymer glass transition temperature). He concluded that a single mechanism involving varying degrees of particle swelling along with solubilization when the copolymer hydrophilicity is high enough explains the thickening transition for all acid containing latices. Once again, a manifestation of this investigation was the occasional observance of a viscosity spike at intermediate degrees of neutralization similar to what Murio and Yudelson had reported. A viscosity spike was one of the three basic shapes designated. AbsentfromVerbrugge's work was the effect of ASE concentration noted by Fordyce and Yudelson. Finally, although no data was shown in his papers, Verbrugge stated that the viscosity offreshlyprepared ASE solutions decreased with time at all pH's, and required at least one day to fully equilibrate. More recently, Quadrat etal (6) examined the swelling and dissolution behavior of ethyl acrylate/methacrylic acid ASE copolymers during neutralization. For these copolymers they reported that acid contents less than 20% only swell, between 2040% they may swell but decompose to smaller units of supermolecular aggregates, and at 40% and above, they may initially swell but are molecularly dissolved. Here again, concentration dependent viscosity maxima were observed both above and well below 100% neutralization depending on the M A A concentration. With increasing acid content, the height of the maxima decreased and was shifted to lower degrees of neutralization. Another profound observation was the effect of flow rate on the viscosity spikes. As the velocity gradient increased, the viscosity maxima decreased and the peaks eventually disappeared altogether beyond some maximum flow. The explanation was decomposition of swollen disperse particles to a system which predominantly contains supermolecular aggregates and only a small proportion of dissolved macromolecules. Several investigators (7-/5) have used potentiometric and conductometric titration techniques to determining the effects of carboxyl group distribution within the latex particles on alkali-swelling behavior of carboxyl functional copolymers. The effect of acid distribution on dissolution behavior was found to be a function of acid type, comonomer type, monomer ratios, copolymer Tg, polymerizaton procedure and the method of monomer addition.



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In this paper, the viscosity of a model HASE polymer exhibiting the viscosity spike previously observed in some ASE's was measured as a function of extent of neutralization in order to gain insight into the mechanism for transition from a low viscosity latex to a relatively high viscosity polymer solution. The types of measurements described in this study were primarily dynamic in nature, thus a particularly unique aspect of this paper is the viscosity equilibration of the model HASE under varying conditions within a reaction calorimeter. It is apparentfromthe literature survey above that this is an area that has received relatively scant previous consideration.

Experimental HASE Thickener Selected for Study The HASE thickener prepared for this study is referred to as HASE-EO40NP and has the general structure shown in Figure 1 as described elsewhere (14,15). The specific ratio of monomers for HASE-EO40NP is W = 60, X = 35, and Y = 5 as a weight percent of the polymer. The polymer was prepared in a 3 liter jacketed glass reactor as a latex at 25% solids by standard emulsion polymerization techniques using anionic surfactant and persulfate initiation. The polymer was characterized by GPC (THF solvent, polystyrene standards) to have a number average molecular weight of 25,000 and a polydispersity index of 2.1. The contrasting hydrophobicity of this polymer in acid form relative to it's very hydrophilic Na+ salt as determined by water vapor sorption has been previously reported (16).

Figure!. Structure of HASE-EO40NP.


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Dynamic Viscosity Measurements A Mettler RC1 calorimeter (reactor type AP01) was used to continuously monitor the viscosity of the HASE thickener during neutralization. The reactor was equipped with an anchor impeller, pH probe, thermocouple, calibration element, and two feed lines. The feed lines were positioned above and below the pH probe to allow for uniform mixing of the sodium hydroxide solution during the neutralizations. During the experiments, the agitator was run at 75 RPM and the temperature was controlled at 25±0.5°C. The rate of agitation was selected by trial and error to provide the optimum balance of uniform mixing and laminar flow over the broadest range of experimental conditions. The Mettler RC1 provides a convenient means for estimating the viscosity of a solution through the storage of a variable called "Rj„" which is the proportional part of the Proportional Integral stirrer controller and is directly related to the required power to achieve a set rate of agitation. While this variable does not directly correspond to the viscosity of the solution being agitated, it is indirectly related to the torque on the stirrer motor, which in turn is directly related to the viscosity of the solution. In fact, it has been shown that under most conditions the R^ parameter is directly proportional to the torque on the agitator (i 7). The RC1 was calibrated using silicon fluids with viscosities of 9.9, 97.5, 985, and 12640 cP (from Brookfield). Experiments were performed where equivalent volumes of these materials were added to the RC1, and the R^ value was obtained at 75 RPM and 25°C. For each standard, Ι^ was determined at two different volumes, 1.15 and 1.25 liters. These volumes correspond to the minimum and maximum volumes present at any time during the neutralization experiments. Using this data, an empirical relationship relating R^to viscosity and reactor volume was developed. ηι

External Measurements In certain cases samples of the partially neutralized HASE Thickener were removed from the calorimeter and characterized for rheology, particle size, and turbidity. The rheology of these samples was detemiined using a Bohlin rheometer operated at 25°C using a cup and bob sample cell. The particle size of the samples was determined using a Microtrac UPA particle sizer. This instrument operates under the principles of dynamic light scattering. The turbidity of the samples was determined using an ANALITE portable nephelometer (Model 156, McVan Instruments).

Results and Discussion Effect of Concentration of HASE Thickener Experiments were performed with different concentrations of HASE-EO40NP in water. The concentrations investigated were 5, 7.5, and 10% at the start of the neutralization. To each of these latexes, 25% sodium hydroxide solution was added such that the neutralization rate and final degree of neutralization was equivalent. The viscosities of the HASE solutions were then plotted as a function of degree of neutralization and pH. To obtain the degree of neutralization, the equivalence point



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was determined from a plot of pH vs. grams of NaOH added. This equivalence point agreed fairly well with that calculated using a mass balance. In these experiments, the HASE latex was neutralized with an excess of NaOH, and therefore degrees of neutralization greater than 100% are reported, although it is recognized that it is physically impossible to neutralize beyond 100%. Figure 2 shows the viscosity of the differently concentrated HASE-EO40NP solutions as a function of extent of neutralization. The data in this figure shows three different types of behaviors. For 10% HASE-EO40NP, a very large increase in the viscosity is observed, followed by a peak and a decrease to a constant value. For 7.5% HASE-EO40NP, the same behavior is observed, but the peak viscosity is much closer to the final plateau viscosity. For both of these latexes the peak viscosity is obtained at about 50% neutralization, or alternatively at a pH of about 6.5. Finally, for 5% HASE-EO40NP a gradual increase in viscosity is observed beginning at about 30% neutralization and ending with a plateau viscosity at about 80% neutralization.

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Percent Neutralized

Figure 2. Viscosity plotted as a function ofpercent neutralization for 5, 7.5, and 10% HASE-EO40NP solutions in water. The rate of neutralization was 50 grams 25% NaOH/hour.

Although all of the references sited in the introduction (2-/5) deal specifically with neutralization of ASE thickeners whereas this work concerns the neutralization of a HASE thickener, the curves in Figures 2 bare striking resemblance to many of those reported previously. Therefore, it seems likely that the mechanism for the latex to polymer-solution transition for HASE thickeners is similar or identical to that for ASE thickeners, and interpretation of the curves in Figures 2 can be aided by consideration of the previous work.


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Effect of Rate of Neutralization The preceding section showed the effect of degree of neutralization on viscosity at a constant rate of neutralization. It is important to note that in the preceding study the HASE thickener is being continuously neutralized and therefore the value of the measured viscosity at any given time is the "instantaneous viscosity" which may be quite different than the fully equilibrated viscosity. In other words the preceding study, like all others performed using this technique, is dynamic in nature. Two different approaches were used to investigate the nature of the equilibration dynamics of HASE-EO40NP. In this section the first of these two approaches is discussed, neutralization of 10% HASE-EO40NP by feeding 25% NaOH at different feed rates. In order to accomplish this, 100 grams of 25% NaOH solution was fed over 0.5, 2, 4, 8, and 24 hours, corresponding to neutralization rates of 200, 50, 25, 12.5, 4.16 grams 25% NaOH solution per hour. Figure 3 shows die viscosity of 10% HASE-EO40NP plotted against the amount of base added for the five different feed rates described above. This figure begins to show the importance of the dynamics of the neutralization process, and the utility of the MetUer RC1 to provide insight into the equilibration kinetics. Figure 3 shows that for degrees of neutralization less than 50% (corresponding to the viscosity peak) as the neutralization rate is decreased the measured viscosity is increased. This suggests that the rate of neutralization is important, and the system is not at equilibrium. On the other hand, Figure 3 also shows that for degrees of neutralization greater than 50% the effect of feed rate on viscosity is less pronounced. These trends can be seen more clearly in Figure 4, which shows the viscosity as a function of feed rate at 37.5%, 50%, and 62.5% neutralization (corresponding to 30, 40, and 50 grams 25% NaOH fed, respectively). By extrapolation, this figure can also be used to estimate the equilibrated viscosity which corresponds to the zero feed rate (fully equilibrated) viscosity. Figure 4 shows that at 37.5% neutralization the viscosity increases with

2750

% Neutralized

Figure 3. Effect of rate of neutralization on the viscosity as a function of degree of neutralization for 10% HASE-EO40NP aqueous solutions.



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decreasing feed rate for all of the rates investigated, with the viscosity rising rapidly when the feed rate approaches zero. This demonstrates that even for the slowest feed rate investigated (4.16 grams NaOH per hour) the HASE polymer does not have time to fully equilibrate. At 50% neutralization this behavior is also apparent, however, at this extent of neutralization the viscosity appears to become constant when the feed rate is less than 25 grams/hour. In other words, for feed rates less than 25 grams per hour, the measured viscosity is independent of feed rate and thus should be the same as the fully equilibrated viscosity which would be measured at a feed rate approaching zero. Finally, at 62.5% neutralization, the viscosity is almost independent of the feed rate. This suggests that at higher degrees of neutralization the equilibration is essentially instantaneous and the measured viscosity is close to the fully equilibrated viscosity.

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The different equilibration behavior at different degrees of neutralization is probably due to the mechanism for thickening at the different extents of neutralization. Specifically, at 37.5% neutralization HASE-EO40NP is opaque and therefore is most likely comprised of water-swollen latex particles. At 67.5% neutralization, HASE-EO40NP is completely transparent and therefore is most likely a polymer solution. In contrast to these degrees of neutralization, at 50% neutralization the physical state of HASE-EO40NP is not as obvious. This is because while HASE-EO40NP turns transparent between 40 and 45% neutralization, the viscosity continues to increase until a maximum at about 50% neutralization. Therefore, at 50% neutralization it is not clear whether the HASE thickener has begun to dissolve or is simply behaving as a highly-swollen latex.


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Viscosity Relaxation of HASE Thickener at Different Degrees of Neutralization The above results showed that the neutralization rate had a pronounced effect on the instantaneous viscosity of HASE-EO40NP, and this effect differed depending upon the extent of neutralization. In this section an alternative approach for studying this behavior is described whereby HASE-EO40NP is neutralized at a constant rate (25 grams/hour) until a desired amount of base is added, after which the feed is stopped and the latex is allowed to equilibrate. The viscosity response during this equilibration is then followed, and the percent change in viscosity and equilibration time are determined. This behavior is referred to as the "viscosity relaxation" of HASE-EO40NP, since the non-equilibrated material is relaxing to its equilibrated state. After equilibration the partially neutralized HASE-EO40NP was removed from the reactor and analyzed for rheology, particle size, and turbidity. Combined, these different measurements were used to infer the mechanism for thickening of HASEEO40NP. Figure 5 shows the relaxation behavior for HASE-EO40NP neutralized to 25, 31.25, 37.5,43.75, 56.25 and 125% neutralization. For all of these experiments 25% NaOH solution was fed at a rate of 25 grams/hour until the desired amount of base was added to the latex, after which the feed was stopped and the HASE was allowed to equilibrate. Figure 6 shows that in each case after the feed was stopped the viscosity changed during an equilibration period, after which it remained constant. The percent change in viscosity and time for equilibration are more clearly illustrated in Figure 6. This figure clearly shows that at all degrees of neutralization the HASE thickener was not instantaneously equilibrated, however, the time and amount of viscosity change upon equilibration was highly dependent upon the actual degree of neutralization. Specifically, between 25 and 37% neutralization HASE-EO40NP requires the greatest amount of time to equilibrate and undergoes the greatest change in viscosity upon equilibration. For the discrete data points measured, at 37.5% neutralization, HASE-EO40NP required greater than 45 minutes to equilibrate and underwent a 175% increase in viscosity during the equilibration. The data in figures 5 and 6 complement the previous data shown in figures 3 and 4 and suggest that the equilibration of a HASE thickener is highly dependent upon its physical state in solution. It appears that at lower degrees of neutralization a significant amount of time is required to achieve a fully equilibrated state, and this behavior is not seen at higher degrees of neutralization. Based on figures 5 and 6 it can further be stated that the longest equilibration times are observed during a very narrow region of degrees of neutralization, apparently when the HASE thickener begins to rapidly swell with water. Further proof of this can be provided by examining Figure 7 which shows the measured particle size and turbidity of the fully equilibrated samples removed from the reactor. This figure shows that the particle size increases rapidly and the turbidity decreases rapidly up to about 37.5% neutralization, which corresponds to the maximum equilibration time. Beyond this degree of neutralization, the viscosity continues to increase and the turbidity continues to decrease, but the equilibration time decreases rapidly. It should be noted that although no particle size could be measured for degrees of neutralization greater than 37.5%, this may be due to limitations in the particle size instrument rather than a true reflection of the physical state of the HASE thickener.


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Figure 7. Turbidity and particle size offully equilibrated samples of 10% HASEEO40NP aqueous solutions at different extents of neutralization.

The data presented in Figure 7 show an important effect of degree of neutralization on the thickening mechanism, and in turn on the rate of equilibration. However, thus far the data are not able to unambiguously show the physical state of the thickener as a function of extent of neutralization, which is important for inferring what the actual mechanism for thickening is. To this end, further information was obtained by analyzing each of the samples discussed in Figures 5 through 7 above for their rheology using a Bohlin Rheometer. Figure 8 shows the rheology of HASEEO40NP at each of the degrees of neutralization discussed above, 25, 31.25, 37.5, 43.75, 56.25 and 125% neutralization. Since HASE-EO40NP exhibits shear thinning behavior in its latex form and nearly Newtonian behavior as a polymer solution, the Bohlin Rheometer enables one to determine at what point the thickener begins to resemble a polymer solution in viscoelastic properties. Figure 8 shows that this point is clearly at degrees of neutralization greater than those corresponding with the peak viscosity. This suggests that up to and around the peak viscosity, the system is comprised of discrete, highly water-swollen latex particles, whereas after the peak viscosity the system is comprised of more or less homogeneous polymer solution. Thus, we conclude that a major cause of the peak viscosity is simply hydrodynamic crowding of the swollen latex particles. This is somewhat different than the mechanism proposed by Yudelson and Mack (3) and later elaborated upon by Nishida et al. (12) who proposed that the cause of the peak viscosity was more due to hydrogen bonding forces than hydrodynamic volume effects.

Validation of Experimental Technique The viscosity data presented in this paper were all obtained using a rather unconventional technique, namely, calibration of a reactor calorimeter not specifically designed for viscosity measurements. While it is obvious that a great deal of information was obtained from this instrument, the precision of this technique has not yet been established which must bring into question some of the results


349

discussed above. To address this, the Bohlin rheometer data shown in Figure 8 was compared with the corresponding viscosities detenmned using the calorimeter. Since the samples were not Newtonian in shear behavior, a linear least squares regression was used to estimate the average shear rate in the calorimeter. The results of this analysis revealed that the average shear rate in the calorimeter was approximately 29 s", and the Mettler and Bohlin measured viscosities were within 5% of each other. Thus, it appears that when properly calibrated the Mettler RC1 can be accurately used to estimate the viscosity of a solution. Downloaded by STANFORD UNIV GREEN LIBR on September 22, 2012 | http://pubs.acs.org Publication Date: August 10, 2000 | doi: 10.1021/bk-2000-0765.ch020

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Conclusions Viscosity measurements performed using an automated reactor calorimeter allowed for precise monitoring of the viscosity of a HASE thickener as a function of degree and rate of neutralization. Using this technique, the mechanism for the dissolution of a HASE latex was inferred, and the importance of the equilibration as a function of degree of neutralization was established. It was found that the neutralization behavior of a HASE thickener is similar to that reported previously for ASE thickeners. Specifically, the viscosity of any given thickener is a function of degree of neutralization and concentration of the thickener. Furthermore, at sufficiently high concentrations a pronounced viscosity spike is observed at degrees of neutralization between 45 and 55%. Rheology measurements suggest the HASE thickener to be comprised of discrete polymer particles at this viscosity peak, thus the cause of this peak appears to be predominantly due to the large increase in hydrodynamic volume of the latex particles in their highly water-swollen state immediately prior to their dissolution. Dynamic measurements revealed that the rate



350

of equilibration of a HASE thickener is a strong function of degree of neutralization, with the longest equilibration times occurring between 25 and 44% neutralization. External turbidity, particle size, and rheology measurements showed that during this narrow range of degrees of neutralization the HASE latex rapidly swells with water and accordingly rapidly increases in diameter. Thus it appears that the long equilibration times between 25 and 44% neutralization are caused by a resistance to swelling, either due to slow diffusion of alkali through the outer layers of hydrated latex, or due to viscoelastic resistance of the polymer chains to unentangle from their initial latex form.

References 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15. 16. 17.

Shay, G. D. In Polymers in Aqueous Media; Glass, J. Edward., Ed.; ACS Advances in Chemistry Series No. 223; American Chemical Society: Washington, D.C., 1989; pp. 457-492. Fordyce, D. B.; Dupre', J.; Toy, W. Offic. Dig. 1959, 31, p. 284. Yudelson, J. S.; Mack, R. E. J. ofPol. Sci.: Part A 1964, 2, 4683-4695. C.J. Verbrugge J. Appl. Polym.Sci.1970, 14, 897-909. C.J. Verbrugge, J. Appl. Polym.Sci.1970, 14, p. 911-928. Quadrat, O.; Mrkvickova, L.; Jasna, E.; Snuparek, J. Colloid & Polym. Sci., 1990, 268, 493-499. Muroi, S.; Hosoi, K.; Ishikawa, T. J. Appl. Polym. Sci. 1963, 11, p. 1967. Muroi, S. J. Appl. Polym. Sci. 1966, 10, 713-729. Hoy, K. L. J. Coat. Tech. 1979, 51, No. 651, p. 27. Bassett, D. R.; Hoy, K. L. In Polymer Colloids; Fitch, R. M., Ed.; Plenum Press: New York, NY, 1980; Vol. II pp. 1-25. Nishida, S., Ph.D. thesis, Lehigh University, Bethlehem, PA, 1980. Nishida, S.; Hosoi, K.; Klein, Α.; and Vanderhof, J. W.InEmulsion Polymerization; ACS Symposium Series No. 165; American Chemical Society: Washington, D.C., 1980; pp. 1291-1314. Loncar, F. V.; El-Asser, M. S.; Vanderhoff, J. W. Polym. Mater. Sci. Eng. 1985, 52, 299-303. Shay, G. D.; Rich, A.F. Journal of Coatings Technology 1986, 58, No. 732, 4353. Shay, G. D.; Bassett, D. R.; Rex, J. D. JOCCA, Surface Coatings International 1993, 76, No. 11, 446-453. Shay, G. D.; Olesen, K. R.; Stallings, J. L. Journal of Coatings Technology, 1996, 68, No. 854, 51-63. Groth, U, Presentation Notes, Rint - Viscosity - Torque; 4 RC User Forum, October 7-10 , Montreux (CH), 1990. th

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